The present invention relates generally to cardiac imaging and, in particular, acquiring cardiac images having minimized motion artifact with high-speed imaging devices. The invention relates to a method and apparatus for acquiring these cardiac images by prospective gating based in part on the length of a cardiac cycle, gender of the subject being imaged and the imaging speed.
Acquiring clear images of the heart is typically impeded by cardiac motion and coronary artery motion caused by the rhythmic beating of the heart. The resulting loss of resolution causes blurring or streaking, called motion artifact, which diminishes the diagnostic value of these images.
Efforts have been made to minimize cardiac motion artifact. Scanning protocols may be adapted to attempt to limit the motion reflected in the cardiac image. Alternatively, patients may simply be instructed to lie still and hold their breath during the scan to reduce respiratory motion, or patient restraints and supports may be used to limit general body motion. In addition, some attempts have been made to shorten image acquisition time by actively increasing the subject""s heart rate and/or increasing the speed of the image-acquisition scan generated by the cardiac imaging device. As an alternative, ECG gating techniques have developed which involve equipping the cardiac imaging apparatus with an ECG gating device that synchronizes the image-acquisition scans with specific phases of the cardiac cycle.
ECG gating relies on the electrocardiographic signals that represent the rhythmic contraction of the heart""s atria and ventricles. These signals originate from electrical pulses of the sinoatrial (SA) node, which spread over the atria and ventricles and cause them to contract, resulting in a complete cycle of the heart""s contractions. Thus, a recorded ECG waveform represents the cardiac cycle, and is comprised of a set of discrete electrocardiographic signals corresponding to the muscular contraction and relaxation of the atria and ventricles. Specifically, the R-R interval measures the period of the heart beat, the P-R segment corresponds to the time from the onset of atrial contraction to the onset of ventricular contraction; the R-T segment approximately measures ventricular contraction or systole; and the T-R interval measures ventricular relaxation, or diastole.
Many known ECG gating techniques involve xe2x80x9cretrospective triggersxe2x80x9d that coordinate the scanning of images with different electrocardiographic signals of the cardiac cycle to obtain a full set of scans over a number of cardiac cycles. Then, the scans are xe2x80x9csortedxe2x80x9d by computerized means according to the phase of the cycle during which they were taken to construct separate images of each phase of the cardiac cycle. Thereafter, the technician or physician selects the clearest image from this series with the least motion artifact for diagnostic purposes.
ECG gating techniques also involve efforts to xe2x80x9cprospectively triggerxe2x80x9d an image-acquisition scan starting with a specific phase of the cardiac cycle, typically at 40-50% of the R-R interval and at 70-80% of the R-R interval. These percentages allegedly correspond to quiescent points of the cardiac cycle where cardiac motion is at a minimum.
There are several problems associated with known ECG gating techniques. First, the traditional techniques generate scan triggers at pre-determined, fixed percentages of the cardiac cycle regardless of the heart rate of the subject being imaged during the scanning procedure. Quiescent points, however, vary with heart rate, so that using a fixed percentage for all subjects regardless of heart rate is ineffective.
Moreover, with many known techniques, the clarity of the resulting images depends on the type of imaging device being used and the speed of the image-acquisition scan it generates. For instance, one ECG gating technique involves using 40-50% of the R-R interval to trigger image-acquisition scan for coronary artery screening or coronary angiography with electron beam tomography (EBT), which has an ultrashort image acquisition time (50-100 ms). This technique may not be as effective with scanning devices having longer acquisition times, such as MRI and spiral CT scanners (100-500 ms).
Finally, because the traditional techniques generate triggers at pre-determined percentages of the R-R interval, the length of each cardiac cycle must be the same during the scanning procedure so that the image-acquisition scan is triggered at precisely the right time for each heart beat. Thus, these techniques produce images with minimized motion artifact only when the heart being imaged has a consistent heart rate, usually measured in beats per minute. ECG gating is not effective for those subjects that have irregular heart rates, or whose heart rates increase or decrease during the imaging procedure, either because of a physical condition or disease or because of stress resulting from the imaging procedure.
Thus, it is an object of the present invention to provide a method and apparatus for acquiring diagnostically valuable cardiac images of the heart having minimized motion artifact via prospective gating by triggering an image-acquisition scan starting at a point of a cardiac cycle, where this point is calculated, in part, by the length of the cardiac cycle. It is yet another object of the present invention to acquire cardiac images having minimized motion artifact with imaging devices having a wide range of scan speeds. Finally, it is an object of this invention to acquire these diagnostically valuable cardiac images in a manner wherein the quality of the resulting images is not dependent on consistent heart rate and, in fact, may dynamically vary as necessary with each heart beat.
The present invention relates to a method and apparatus for acquiring an image of the heart by triggering an image-acquisition scan starting at a point of the cardiac cycle having minimized motion. The optimal point for triggering an image-acquisition scan is determined by measuring the lengths of the cardiac cycles of the subject being imaged. The optimal trigger point of each cardiac cycle depends on the length of that cycle; thus, this point varies as the length of the cardiac cycle changes. Thus, preferably, the present method and apparatus measures each cardiac cycle during the scanning procedure and dynamically determines and adjusts the optimal trigger point from one heart beat to the next as necessary. Thus, in the preferred embodiment, the diagnostic value of the images obtained pursuant to this invention does not rely on the subject having a constant heart rate throughout the duration of the scanning procedure.
The method disclosed by the present invention acquires a cardiac image with minimized motion artifact by measuring the length of the R-R interval of a particular heart beat, calculating the length of the R-T segment to determine the quiescent segment of the cardiac cycle, identifying an optimal scan starting point of the cardiac cycle, and triggering an image-acquisition scan at this point.
The calculation of the R-T segment length is based in part on the gender of the subject and the length of the R-R interval. For men, the R-T length is calculated by the algorithm 0.143xc3x97RR+224.2; for women, the R-T length is calculated by the algorithm 0.157xc3x97RR+221.2. In both cases, the R-R interval=1000 msxc3x9760/heart rate (ms). The quiescent segment, which corresponds to the period of minimized cardiac motion velocity, is late systole to early diastole, and approximates the end of the R-T segment.
The optimal scan starting point, which is within the quiescent segment, is based on the image-acquisition scan speed and the subject""s heart rate. The present method contemplates the use of scan speeds that fall within the range of about 25 ms to about 250 ms. As an example, the optimal scan starting point is at 25-50 ms before the end of the R-T interval where the speed of the scan protocol is 25-100 ms for any heart rate. However, this scan starting point is suboptimal where the scan protocol speed is over 150 ms and the subject""s heart rate is less than 61 beats per minute. In this case, mid-diastole provides for better scan points because the longer quiescent segment is at 60-80% of the R-R interval. Thus, the optimal scan starting point will vary with each subject""s heart rate and the imaging device used. The optimal scan starting point in the R-R interval is identified by the algorithm RS=RTxc2x1X, where RS refers to the length of time from the peak of the R wave to the scan starting point, the length of the RT segment is determined by the formulas set forth above, and the xe2x80x9cXxe2x80x9d value depends on the scan speed, as listed in table 1 herein.
The present method is implemented by a cardiac imaging apparatus that is capable of generating image-acquisition scans within the range of about 15 ms to about 300 ms, namely MRI devices, spiral CT scanners and EBT scanners. The apparatus comprises a transmitter that generates the image-acquisition scan, an input console that receives parameters used in implementing the above method, and an ECG gating device that is connected to and adapted to communicate with the transmitter and the input console. This gating device synchronizes the triggering of image-acquisition scans with specific points of the cardiac cycle. The ECG gating device includes hardware that receives electrical signals representing the cardiac cycle and triggers image-acquisition scans. The ECG gating device also preferably includes software that operates the gating hardware and is adapted to implement many of the steps described above, i.e., to measure the length of the R-R interval, calculate the length of the R-T segment and to identify the optimal scan starting point. Alternatively, these steps towards identification of the optimal scan starting point may be implemented manually by the physician or technician overseeing the scanning procedure who can relay the calculated optimal scan starting point to the ECG gating device via the input console. Regardless of how this optimal scan starting point is relayed to the gating hardware, it electronically triggers the transmitter to release an image-acquisition scan at that point.